Electron gun suitable for electron microscope



United States Patent US. Cl. 315-45 Claims ABSTRACT OF THE DISCLOSURE Toprevent positive ions from hitting the filamentary cathode in anelectron beam gun and damaging it, the electron beam is hollowed out andan electrode is positioned in the hollow to stop the flow of thepositive ions back to the cathode.

This invention relates to an electron beam producing means, for example,an electron gun, which produces a straight electron beam and includes anion trap along the axis of the beam. The beam producing means providedby this invention is particularly suitable for use in an electronmicroscope but is not limited to that use.

As an electron beam producing means or gun operates to produce a highvoltage electron beam, undesirable ions are also being produced whichare accelerated by the same field that accelerates the electrons of thebeam but in the opposite direction. These ions hit the cathodecomprising the source of electrons for the beam and may damage oreventually destroy it. A known method of preventing the ions fromhitting the cathode is to use a bent beam. However, use of such a beamin an electron microscope, for example, causes asymmetrical deflectiondefocusing of the beam which degrades the electron microscope imageproduced by such a beam. Also, when a beam is accelerated, as byaccelerating electrodes, the voltage applied to the acceleratingelectrodes has a lens effect on the beam, whereby a change inaccelerating voltage requires a change in focusing of the beam.

It is an object of this invention to provide an improved straightelectron beam producing apparatus including an in-line or axial iontrap.

It is another object of this invention to provide an electron gunincluding such anion trap.

It is still another object of this invention to provide an electron beamsource for a linear electron accelerator in which defocusing of the beamby changes of acceleration voltage is minimized.

In accordance wit-h this invention, lens means is pro vided forhollowing out an electron beam in a symmetrical manner with respect tothe axis of the beam and for returning the electrons of the beam totheir beam form, to produce an electron free space in the middle of thebeam. An ion trap is positioned in the electron free space in the beam.Other lens means are provided for further focusing the beam and meansare provided for accelerating the electrons thereof, the other lensmeans and the accelerating means including an element in common. Whilethe electron beams so provided can be used for many purposes for whichan electron beam gun is suitable, such as the beam gun of a cathode raytube, it can also be used in a ice high voltage beam microscope withfurther beam accelerating means. When so used, the defocusing of thebeam resulting from changes in voltage of the further acceleration meansis minimized by the construction of the electron gun.

The invention may be better understood upon reading the followingspecification in connection with the accompanying drawing in which thesole figure is a fragmentary section of an electron microscope takenthrough a vertical axis thereof.

Turning to the figure, a filamentary cathode 10 which is heated and heldat a high negative accelerating potential with respect to ground bysuitable means, not shown, acts as a source of electrons for the beam12. A grid cap 14 which is held at a bias voltage such as 45 volts withrespect to the filamentary cathode 10' surrounds the cathode 10. Thegrid cap 14 is symmetrical about its central vertical axis as viewed inthe figure. The central portion of the cap 14 is in the form of a plate16 having a hole 18 therethrough for the passage of electrons from thecathode 10 and the outer portion of the grid cap 14 is formed to providean outwardly and downwardly slanting conical wall 20. A further axiallysymmetrical electrode 22 which comprises an annular shaped disk 24 and acylindrical portion 26 is positioned coaxially with the cap 14 and withthe disk 24 below the cap 14, the cylindrical portion 26 surrounding thegrid cap 14. The electrode 22 is maintained by suitable means, notshown, at the voltage of the cathode 10. An axially symmetricalelectrode 28, which is maintained at about 6,000 volts positive withrespect to the cathode 10 by suitable means, not shown, is alsopositioned coaxially with the cap 14. The electrode 28 comprises ahollow portion 30 which is frusto-conical both at its outer and innersurfaces, the tip of the portion 30 extending up into the conicalportion 20 of the grid 14. The upper end of the portion 30 has a holetherethrough for passage of electrons. The lower part 32 of the conicalportion 30 is cylindrical in internal shape. The conical portion 30 isjoined to an annular disk-like portion 34 by an intermediate portion 36.The lower edge of the intermediate portion 36 is hollowed out as shownto provide clearance between the electrode 28 and an electrode 38 aswill be more fully explained.

Due to the shape of the lower surface of the grid cap 14 and to theshapes of the electrodes 22 and 28 and due to the potentials appliedthereto, the beam of electrons 12 exiting through the hole 18 willexpand radially in a symmetrical manner as it goes down and willtherefore take the conical shape shown, the cylindrical portion 32cansing a speeding up of the radial expansion of the beams since thesurface 32 does not recede from the beam as does the internal conicalsurface portion just above it. This expanding portion of the beam 12acts as if it originated at the point 40, at the apex of the linescomprising the extensions in an upward direction of the envelope of thebeam 12. Means are provided to hollow out the beam 12 and then tocontract it in a radial direction. The beam hollowing out means includesthe axially symmetrical electrode 38 which is maintained at the samevoltage as the cathode 10 by suitable means, not shown. The electrode 38includes a pointed portion 42, the point thereof extending upwardly, theportion 42 being supported by spider arms 3 44 from an annular disk 46comprising another portion of the electrode 38.

While only two spider arms 44 are shown, there may be as many spiderarms (three for example) as is desired. The outer portion of theelectrode 38 is turned up and surrounds the outer edges of the disk 34forming part of the electrode 38. The inner edge of the disk 46 isformed into an upstanding portion 48 whose inner shape is cylindricaland whose outer shape is conical and slants outwardly in a downwarddirection. The lower portion of the disk 46 is formed to receive aportion of an electrode 52 as will be explained. Since the electrode 38is at a high negative potential, that of the cathode 10, and since theelectrode 28 is at a high positive potential with respect thereto, theportion 36 of the electrode 28 is cut away to prevent arcing between theelectrodes 28 and 38. Also the turning up of the outer potrion of theelectrode 38 to surround the electrode 28 is to prevent arcing betweenthe electrode 28 and the microscope casing, not shown, of which thedescribed electron gun may be a part. The shape and position of thepointed electrode portion 42, and the negative voltage applied to it,causes the electron beam to be hollowed out to provide a substantiallyelectron free space within the beam 12 immediately surrounding thepointed portion 42, the space so formed being symmetrical with respectto the axis of the beam 12.

The means for causing the beam 12 to contract in a radial directionincludes the electrodes 38, 50 and 52. The electrode 50, which is alsoaxially symmetrical, includes an outer annular disk portion 54, atransitional portion 56, spider arms 58 and a central disk potrion 60.The transitional portion 56 has a conical upper surface and a moresteeply conical lower surface and joins the lower annular portion 50 tothe spider arms 58 which support the disk 60. While only two arms 58 areshown, there may be as many thereof (three for example), as may bedesired.

The electrode 50 is maintained at the same high positive potential withrespect to the cathode as the electrode 28 by suitable means, not shown,whereby the arms 58 are also highly positive, and whereby the electronsof the beam 12 are attracted thereby. To reduce attenuation of the beam12 by the arms 58, the arms 58 are positioned in the shadow of the arms44 which are at the same potential as the cathode.

The electrode 52, which is held at the potential of the cathode 10 bysuitable means, not shown, comprises an annular disk 62 having an innerdiameter equal to the inner diameter of the central upstanding portion48 of the electrode 38 surrouding the beam 12 and an upstandingcylindrical portion 64 having the same inner diameter as the disk 62 anda much smaller outer diameter. The cylindrical portion 64 extends upinto and fits into the electrode 38 and is electrically connectedthereto, to form with the inner surface of the portion 48 a longequipotential cylindrical surface. To provide clearance for the spiderarms 44 and 58, vertical slots 66 are cut in the upper end of thecylinder 64. While no insulation is required for the arms 44, the arms58 and the portion 56 of the electrode 50 are insulated by vacuum fromthe cylinder 64. Due to the voltages applied to the electrodes 38, 50and 52, the electrons are attracted inwardly by the positive voltage onthe disk 60, and the electrons are also repelled inwardly by thenegative voltage on the inside surface of the electrodes 38 and 52 tocause the beam 12 to consolidate and to cross over at the point 68 andto become a solid beam. The electrons of the beam 12 go downward fromthe point 68 so that they appear to originate at the point.

70 which is the point of intersection of straight lines tangent to theenvelope of the beam. An axially symmetrical electrode 72 is positionedcoaxially with the electrodes 28, 38, 50 and 52. This electrode 72,which is maintained at a positive potential of about 560 volts withrespect to the cathode 10 by suitable means, not shown, comprises anouter annular disk 74 in an inner annular disk 76 connected by atransitional portion 78. The disk potrion 76 extends up into the lowerpart of the annular disk 62 to produce the necessary electrostatic fieldshape to cause the beam to appear to come from point 70. The hole in thedisk 76 increases in diameter in a downward direction in a known manner.The beam 12 passes through the hole in the portion 76, and the beamemerging therefrom, due to the electrodes mentioned and the voltageapplied thereto, looks as if it originated at the point 70 with the sameangular divergence and envelope as if it originated from point 40.However, an ion that originates in the portion of the described gunbelow the ion impermeable disk 60 hits the disk 60 and is stoppedthereby. The ion cannot hit and therefore does not injure the cathode10, whereby the life of the cathode 10 is extended by the provision ofthe described ion trap thereof.

The electron beam is accelerated and further focused by the electrode 72in conjunction with the electrodes 80 and 82, whereby the electrode 72acts both as an element of the coaxial ion trap described as well as anelement of a beam accelerator. The axially symmetrical element 80comprises an outer annular disk portion 84 and a centrally positionedupstanding hollow portion 86 having a conical upper surface and acylindrical inner surface. The electrode 80 also includes a downwardlyextending hollow portion 88 having a cylindrical inner surface 90 and acylindrical outer surface 92 adjacent to the disk 84 and an outerconical surface 94 which decreases in diameter in a downward directionjoining the cylindrical surfaces 92 and 90. The diameter of the surface90 is greater than the inside diameter of the portion 86. An apertureddisk 96 closes the top of the cylindrical surface 90. The aperture inthe disk 96 may be biconical as shown. The disk 96 is thinner than thedisk 84 and the lower surfaces of the disks 84 and 96 are in the sameplane. The upper end of the portion 86 of the electrode 80 extends intothe transitional portion 78 of the electrode 72. The voltage applied tothe electrode 80 by suitable means, not shown, may be 660 voltspositive.

The electrode 82, which is axially symmetrical, has a positive voltageof 560 volts applied thereto by suitable means, not shown. Thiselectrode 82 comprises an outer Iannular disk portion 98 having a turneddown rim 100. The electrode 82 also includes a central annular diskportion 102 having a hole therethrough for the passage of electronswhich has a biconical shape comprising a shorter frusto-conical portion,whose diameter decreases in a downward direction, and a longerfrusto-conical portion whose diameter increases in a downward direction,the two smaller diameter portions of the frustums being joined. Theshape of the aperture through the disk portion 102 aids in maintainingfocus of the beam 12. The electrode 82 also includes a transitionalportion 104 having an upper conical surface conforming in angle with thesurface 94 of the electrode 80 and spaced therefrom, the conical surface94 extending into the transitional portion 104. The outer surface of thetransitional portion 104 is cylindrical. The portion 106 of theelectrode 82, which, as shown, may be made up as a separate part fromthe portions 98, and 104 thereof for the purpose of ease of manufacture,fits over the lower end of the cylindrical outer surface of thetransitional portion 104. The portion 106 has a conical inner surface108 whose diameter decreases in a downward direction. An apertured plate110 closes the lower part of the lower portion 106. The aperture in theplate 110 may have a wedge shaped inner edge as shown. X deflectingplates 112 and Y deflecting plates 114, only one of the Y plates beingshown, are provided in the chamber within the conical surface 108 andbetween the apertured disks 102 and 110. By adjustment of potentialsprovided by suitable means, not shown, applied to the deflection platesX and Y, the direction of the beam 12 emerging below the disk 10 may becontrolled.

The device so far described may be used as a complete electron gunrequiring no additional acceleration and including the described iontrap. However, the described gun may also be used as a source of a beamof accelerated electrons subject to further acceleration as in anelectron microscope. When so used, a further acceleration electrode 120may be provided. This electrode 120 comprises a disk aperture forpassage of the electron beam 12 there through, to which high positivevoltage in the order of 6.6 to 33 kv. (kilovolts) may [be applied. Thisaccelerating electrode 120 may be one of a plurality (up to 15) ofsimilar electrodes positioned along the beam 12 below the electrode 120to which successively higher positive voltages are applied, the highestof which may be as high as 500 kv. when the voltage on the electrode 120is 33 kv. The voltage on the accelerating electrodes, of which only theelectrode 120 is shown, while accelerating the beams 12 passingtherethrough also produces an electron lens action which results in adefocusing of the beam 12. Using the described electron gun withaccelerating electrodes such as electrode 120, only the voltage on thegrid cap 14 need be changed to retain focusing of the beam 12, as thevoltage of the accelerating electrodes such as electrode 120 is changed.This is due to the fact that the accelerating electrode 120 and anysubsequent accelerating electrodes such as electrode 120 (not shown)constitute a weak electrostatic aperture lens having a very long focallength. The electrodes 72, 80 and 82, on the other hand, constitute amuch stronger electrostatic lens having a much shorter focal length.Because of the proximity of this strong lens comprising the electrodes72, 80 and 82 to the electrode 120 and to the subsequent acceleratingelectrodes (not shown), the strong lens 72, 80 and 82 is immersed in thefocal length of the weak lens comprising the electrode 120 andsubsequent electrodes (not shown). In this situation a small focusingchange in the stronger lens 72, 80 and 82 can overcome a focusing changedue to a great voltage change in the weaker lens. The small focusingchange in the strong lens 72, 80 and 82 is provided by varying thevoltage in the grid cap 14, without varying the voltages in theelectrodes 70, 80 and 82. Since change of the voltage on the grid cap 14changes the position of the point 70, where the electrons of the beam 12appear to originate, the focusing effect on the beam of electrons of thelens 72, 80 and 82 is changed.

This change of potential on the grid cap 14 is from about 45 volts toabout 60 volts, or about 33%, as the voltage on the electrode 120 ischanged from about 6.6 kv. to about 33 kv. or about 500%. Therefore, theelectron gun described minimizes the defocusing of the beam thereof uponfurther acceleration of the beam and reduces the changes in potential ofthe grid cap 14 necessary to track, that is to keep the beam focused asthe acceleration voltage on the electrodes, such as electrode 120, ischanged.

While the several electrodes (except the cathode which may be made oftungsten) may be made of any solid nonmagnetic conductor, preferablythey may be made of stainless steel.

Many modifications of the above-described invention will occur to aperson skilled in the art. For example, the voltage mentioned and thedetails of the shapes of the electrodes are exemplary only.- The abovedescription is therefore to be considered as illustrative and not in alimiting sense.

I claim: 1. In combination: lens means for expanding an electron beam inall radial directions from the axis of said ibeam along a portion of itslength and for hollowing out an expanded portion of said beam to providean electron free space,

means impervious to the passage of ions in said space to stop ions frommoving along said beam and through said electron free space,

means for contracting the radial diameter of said beam after theexpansion of said beam,

said means for expanding the diameter and for hollowing out the expandedportion of said beam comprising a conical electrode surrounding saidbeam, and

a pointed electrode positioned along the axis of said beam.

2. The invention as described in claim 1 and including:

an electrode for focusing said contracted beam to cause the electrons ofsaid beam to travel in the same direction and at the same speed.

3. The invention as described in claim 1 and including:

an electrode for focusing said contracted beam to cause the electrons ofsaid beam to travel in the same direction and at the same speed, and

an electron accelerating means, said acceleration means,

including said focusing electrode.

4. An electron beam gun comprising:

a cathode having an electron emitting portion positioned along an axis,

a grid cap having a circular aperture surrounding and coaxial with saidaxis,

an axially symmetrical hollow frustro-conical electrode coaxiallypositioned with respect to said axis, the small end of said electrodebeing adjacent said grid p,

an axially symmetrical pointed electrode positioned coaxially withrespect to said axis at a point beyond said conical electrode from saidgrip cap, the pointed portion of said pointed electrode extendingtowards said conical electrode,

an ion impermeable electrode positioned along said axis [beyond saidpointed electrode in a direction away from said cap, and

an axially symmetrical hollow cylindrical electrode positioned coaxiallywith said axis and surrounding said pointed electrode and said ionimpermeable electrode.

5. The invention as claimed in claim 4 in which said cylindricalelectrode extends beyond said ion impermeable electrode in the directionaway from said grid cap.

6. The invention as claimed in claim 4 in which said cylindricalelectrode extends beyond said ion impermeable electrode in the directionaway from said grid cap, and

an additional axially symmetrical focusing electrode positionedcoaxially with said axis and within said cylindrical electrode.

7. The invention as claimed in claim 4 in which said cylindricalelectrode extends beyond said ion impermeable electrode in the directionaway from said grid cap,

an additional axially symmetrical focusing electrode positionedcoaxially with said axis and within said cylindrical electrode,

said gun also including first and second axially symmetricalaccelerating electrodes coaxially positioned with respect to said axisand beyond said focusing electrode,

said second accelerating electrode being more remote from said grid capthan said first accelerating electrode.

8. The invention as claimed in claim 4 and including means to applypositive potential to said conical and said ion impermeable electrodeswith respect to said grid cap and cathode potential to said cylindricalelectrode and pointed electrode whereby the electron stream produced bysaid cathode is hollowed out to produce an electron free space alongsaid axis, said ion impermeable electrode being within said electronfree space.

9. The invention as claimed in claim 7 including means to apply highpositive voltage to said conical and said ion impermeable electrodeswith respect to said grid cap and cathode potential to said cylindricalelectrode and pointed electrode whereby the electron stream produced bysaid cathode is hollowed out to produce an electron free space alongsaid axis,

7 8 said ion impermeable electrode being within said elec- ReferencesCited tron free space, and further including UNITED STATES PATENTS meansto apply low positive voltage to said focusing l t d d to the second oneof said accelerating 1617360 11/1952 De Gler 313 82 electrodes and anintermediate positive voltage to 5 2661436 12/1953 Van Ormer 313*82 saidfirst one of said accelerating electrodes. 2888605 5/1959 Brewer 315 '1510. The invention as claimed in claim 9 and including 3,201,631 8/1965Gale et 250-495 3,307,065 2/1967 Arnaud et al. 315-3l X a third axiallysymmetrical accelerating electrode positioned coaxlally with said axisand beyond said second RODNEY D. BENNETT JR" Primary Examiner.

accelerating electrode in a direction away from said grid 10 cap, and M.F. HUBLER, Assistant Examiner.

means to apply a positive voltage of the order of the voltage applied tosaid conical electrode to said third US. Cl. X.R.

accelerating electrode. 31382

